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Rollup merge of #117039 - scottmcm:clarify-get-unchecked, r=cuviper

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Clarify UB in `get_unchecked(_mut)`

Inspired by #116915, it was unclear to me what exactly "out-of-bounds index" means in `get_unchecked`.

One could [potentially](https://rust.godbolt.org/z/hxM764orW) interpret it that `get_unchecked` is just another way to write `offset`, but I think `get_unchecked(len)` is supposed to be UB even though `.offet(len)` is well-defined (as is `.get_unchecked(..len)`), so write that more directly in the docs.

**libs-api folks**: Can you confirm whether this is what you expect this to mean?  And is the situation any different for `<*const [T]>::get_unchecked`?
This commit is contained in:
Matthias Krüger 2023-11-10 01:50:24 +01:00 committed by GitHub
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library/core/src/slice/mod.rs
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@ -640,6 +640,11 @@ impl<T> [T] {
/// Calling this method with an out-of-bounds index is *[undefined behavior]*
/// even if the resulting reference is not used.
///
/// You can think of this like `.get(index).unwrap_unchecked()`. It's UB
/// to call `.get_unchecked(len)`, even if you immediately convert to a
/// pointer. And it's UB to call `.get_unchecked(..len + 1)`,
/// `.get_unchecked(..=len)`, or similar.
///
/// [`get`]: slice::get
/// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
///
@ -675,6 +680,11 @@ impl<T> [T] {
/// Calling this method with an out-of-bounds index is *[undefined behavior]*
/// even if the resulting reference is not used.
///
/// You can think of this like `.get_mut(index).unwrap_unchecked()`. It's
/// UB to call `.get_unchecked_mut(len)`, even if you immediately convert
/// to a pointer. And it's UB to call `.get_unchecked_mut(..len + 1)`,
/// `.get_unchecked_mut(..=len)`, or similar.
///
/// [`get_mut`]: slice::get_mut
/// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
///